JPS60186427A - Manufacture of optical fiber preform - Google Patents

Abstract

PURPOSE:To suppress the corrosion of a furnace core tube, and to obtain an optical fiber preform at a low cost, by heating a porous preform at a low temperature to effect the thermal shrinkage, and reheating to the vitrification temperature. CONSTITUTION:The porous preform 13 deposited in the reaction vessel 16 made of e.g. Pyrex glass, etc. is heated in the electric furnace 15 maintained at about 1,300 deg.C (lower than the vitrification temperature) to effect the thermal shrinkage and obtain a semitransparent preform 12. The porous preform 13 is dehydrated in the above process with chlorine gas. The semitransparent preform 12 is transferred to another electric furnace, and reheated at about 1,500 deg.C to obtain a transparent glass preform. The reheating process is carried out using a furnace core tube made of carbon in the stream of pure helium gas to minimize the corrosion of the carbon core tube. Since the pressure control in the electric furnace and the core tube is not necessary, the operation can be simplified and the yield of the optical fiber preform can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for producing transparent glass in the production of optical fiber preforms.

(Prior art) Conventionally, in a method for manufacturing an optical fiber base material, when obtaining a transparent glass base material from a porous base material formed by synthesizing glass fine particles and depositing them, the J... It becomes transparent vitrified at elevated temperature.

Figure 1 shows an example of a conventional optical fiber base material production system, in which 1 is a quartz glass seed rod, 2 is a transparent base material,
8 is a porous base material, 4 is a furnace tube made of quartz glass 5, 5 is an electric furnace for transparency, 6 is a reaction vessel made of Pyrex glass, 7 is a synthesis torch made of quartz glass, 8 is a glass raw material supply system, and 9 is a gas The seal portion, IO, is an electric furnace internal pressure gauge. In A and H, the quartz glass furnace core tube is flanged to maintain hermeticity of 1.

Normally, when the outer diameter of the porous base material 8 is about 61 mm, the electric furnace 5 must be heated to 1500'C to make it transparent, and thionyl chloride is required to dehydrate the porous base material. Since a salt-purple removal agent such as chlorine gas is used, a quartz glass furnace core tube 4 had to be used due to poor sealing and processability. However, the quartz glass furnace core tube 4
The operating temperature is up to around 1800°C, and in order to maintain the shape, the internal pressure of the electric furnace must be monitored with the electric furnace internal pressure gauge 10! ...and the appropriate value also changes depending on the pressure inside the reactor core tube, making adjustment of the first part difficult. Furthermore, since it is constantly used at temperatures above the usable temperature, the quartz glass suffers from devitrification and cracking, resulting in rapid wear and tear.

In addition, in recent years, with the mass production of optical fibers, it has become necessary to increase the outer diameter of the porous base material in order to make it more economical. Heat losses due to absorption and the larger furnace diameter increase, leading to higher loading densities on the heating elements. In a tubular electric furnace, the area of the heating part cannot be changed significantly;
As the load density increases, the surface temperature of the heating element increases.

- For example, when transparentizing a porous base material with an outer diameter of 1801Is with a quartz glass furnace tube inserted in a carbon resistance furnace with an inner diameter of 150mm, 8℃/''
A heating part temperature of 1550°C was required with a temperature increase of min.

Therefore, an electric furnace that has a corresponding inner diameter and the ability to raise the temperature to obtain transparent glass at the same time has a large energy loss, and the heating element temperature must be set high, and its power consumption and dimensions 1, it had to be made very large.

Another drawback is that it is difficult to simultaneously equip a porous base material manufacturing apparatus.

(Object of the Invention) In order to eliminate these drawbacks, the present invention heat-treats the porous base material at a temperature at which the volume of the porous base material once shrinks. The purpose of the present invention is to reduce the wear and tear of the tube, eliminate the need for pressure adjustment in the electric furnace and the furnace core tube, simplify the operation, and lower the cost of the base material for optical fibers and, in turn, the cost of optical fibers.

(Structure and operation of the invention) First, the basic concept of the present invention will be explained.

Figure 2 shows the shrinkage rate of the porous base material. This shows the shrinkage rate when a porous base material is cut out and heat treated in helium gas at various temperatures.

When the contraction rate is k, k=Δl/lo= (lo-1)/
l.

I took it. Here! . 211 of the porous base material before heat treatment
(8) It is the length! is the length of the porous base material after heat treatment, which is 1.

Note that I indicates the characteristics when the Geog content in the porous base material is high, and ■ indicates the characteristics when the Geog content is high in the porous base material. The characteristics when the content is small are shown.

As can be seen from FIG. 2, shrinkage begins at about 1000'C, although it varies depending on the concentration, initial density of the porous matrix, and saturates at about 50% shrinkage at 1200-1800'C. After that, it begins to become transparent and reaches approximately 1500℃.
Transparency is completed.

Next, one embodiment of the present invention will be described.

FIG. 8 shows a production system for the optical fiber base material used in this example. Here, 11 is a quartz glass seed rod, 12 is a translucent base material, 18 is a porous base material, 14 is a quartz glass furnace tube, 5 is an electric furnace, 1615 is a Pyrex glass reaction vessel, 17 is a quartz glass A glass synthesis torch, 18 is a glass raw material supply system, and 19 is a glass sealing part. In this example, the electric furnace 15 used was an electric furnace having a carbon heating element with an inner diameter of 1 (l Qwg).The furnace wall was water-cooled, and there was no need to particularly adjust the pressure. The furnace core tube 14 was flanged at A and B to maintain airtightness.

The porous preform 18 deposited in the Pyrex glass reaction vessel 16 was shrunk in an electric furnace kept at 1800° C. and taken out as a translucent preform 12. At this time, in this example, the porous base material was dehydrated by using chlorine gas. The semi-transparent base material 12 taken out was reheated to 1500°C in another electric furnace to obtain a transparent glass base material. At this time, the electric furnace used for this purpose was heated without flowing chlorine gas, using a carbon furnace tube and only flowing helium gas.

Therefore, no wear and tear on the carbon core tube was observed.

The outer diameter of the deposited porous base material 18 was 1δQ ws , and the outer diameter of the semitransparent base material 12 was 64N. Furthermore, the loss characteristics of the optical fiber obtained from the 15-transparent base material were also comparable to those of conventionally produced optical fibers. At this time, the quartz glass furnace core tube 14 can be operated without pressure adjustment, etc.
Even after use, no changes were observed in appearance. In addition, the translucent 2□ base material 12 has a higher degree of sintering than the porous base material 18, and its dimensions are smaller, so operations such as removal were easier (effects of the invention). As explained above, according to the present invention, wear and tear on the quartz glass furnace core tube is reduced, and there is no need for pressure adjustment in the electric furnace and the furnace core tube, making the operation simple, and the progress of the base material for optical fibers has been improved. Since the termination is improved, there is an advantage that it can contribute to lowering the cost of optical fiber.

Similarly, even when the optical fiber base material is synthesized at high speed, continuous sintering can be performed, contributing to cost reduction. Furthermore, these methods can be applied not only to the production of optical fibers but also to the production of glass rods and the like.

[Brief explanation of drawings]

Figure 1 is a diagram showing an example of a conventional optical fiber base material production system, Figure 2 is a diagram showing the relationship between temperature and shrinkage rate of a porous base material, and Figure 8 is a diagram showing an example of an optical fiber base material production system of the present invention. Fiber 20 (7
) It is a diagram showing the production system of the base material. DESCRIPTION OF SYMBOLS 11... Quartz glass seed rod 12... Translucent base material 18... Porous base material Todo... Quartz glass furnace core tube 15... Electric furnace 16... Pyrex glass reaction vessel 17 ...Quartz glass synthetic torch 18...Glass raw material supply system 19...Gas seal section. Patent applicant Nippon Telegraph and Telephone Public Corporation (8) Figure 2 g1 Kaisho GO-186427 (4)

Claims (1)

[Claims] L A base material for an optical fiber in which glass fine particles are generated by reacting glass raw materials, and the glass fine particles are deposited to form a porous base material, which is then heated to a high temperature and sintered to become transparent vitrification. A method for producing an optical fiber base material, which comprises heating and shrinking at a temperature lower than the transparent vitrification temperature and then reheating to the transparent vitrification temperature as a step before transparent vitrification. .